The invention relates to an electrode element for plasma torches as well as a production method for such electrode elements. Such an electrode element is particularly suitable for plasma cutting in which oxygen is used as a plasma gas.
Such electrodes are very highly stressed thermally and electrically when used in plasma torches such that they only achieve limited service lives, and an expensive replacement of electrodes is required in more or less long time intervals.
In particular, the high thermal load caused by temperatures of up to 50,000 K requires an appropriate design and a suitable selection of the materials used for such an electrode.
Thus, up to now for plasma cutting using oxygen as a plasma gas, electrodes substantially made up of hafnium are employed with a melting temperature in the range of 2220° C. Hafnium has a low work function in contrast to many other electrically conducting metals such that it is especially appropriate for the application.
As a rule, such pencil-shaped hafnium electrodes having a copper socket are used, and at the same time use is made of the high thermal and electrical conductivity of copper.
However, having such a formation the electrical anodic corrosion (electromigration) and diffusion, which increases the transition resistance between the hafnium and copper, has to be kept small.
In particular during plasma cutting with oxidizing gases such as the already mentioned oxygen, oxidation occurs with the copper such that this has a bad influence on the thermal conductivity and the electrical transition resistance between the copper and hafnium.
Due to the high anodic corrosion and oxidation, the result is an increased power conversion at the boundaries between the hafnium and the copper such that the aging processes proceed in an accelerated manner.
Because of the enhanced formation of copper oxide on the copper sheath at higher temperatures in close proximity of the hafnium core, the work function of copper is decreased, and, accordingly, copper electrons can also emit out of it. Because of this emission, it may result in local fusing of the copper and accordingly in an unserviceability of such a plasma electrode.
According to the prior art, silver or a silver alloy are used to counteract these problems. Silver has also good thermal and electrical conductivities as well as a higher work function. In particular, the oxide formation with silver is less in contrast to copper at higher temperatures.
An equivalent solution is described in EP 0 980 197 A2. On that occasion, a copper holder finds use into which a silver sleeve made of a selected silver alloy and having a closed bottom facing into the interior of the copper holder is pressed into a receptacle formed as a blind hole.
Then, a pencil-like electrode made of hafnium is again pressed into this silver sleeve.
Such a structure has several disadvantages. This concerns the expensive production, on the one hand, during which the individual elements have to be fabricated separately and partially by metal cutting. The three individual parts have then to be joined together into one element wherein high demands have to be met upon joining and handling because of the relatively small-sized silver sleeve and the hafnium pin. In addition, mechanical pressing of the silver sleeve and the hafnium pin has to be carried out very carefully.
Nevertheless, merely a locally limited contact between the copper, silver and hafnium can be achieved such that in particular these spot-shaped contacts have an adverse effect with respect to the anodic corrosion already mentioned, and, of course, the thermal conductivity is correspondingly negative influenced as well.
Accordingly, with such a solution the service life is negligibly increased in contrast to electrodes that were previously known, and the service life increase for such electrodes used for plasma torches is largely compensated due to significantly higher production costs.
Therefore, it is desirable to propose electrode elements for plasma torches as well as a suitable production method in which the production costs can be reduced with a simultaneous increase of the service life.